A major function of the renal proximal tubule is to secrete acid into the tubule lumen, thereby reabsorbing most of the filtered HCO-3, and exporting the acid load produced by body metabolism. Although it is well established that the proximal tubule (PT) and more distal nephron segments can modulate its rate of acid secretion in response to changes in the acid-base status of the blood, the mechanisms by which this occurs over a short time frame are not clear. We have studied rates of HCO-3 reabsorption (JHCO3) in isolated perfused rabbit PTs, using out-of-equilibrium (OOE) CO2/HCO-3 solutions to vary, one at a time, the 'bath' or basolateral (BL) acid-base parameters. We found that, although JHCO3 is insensitive to isolated changes in pHBL, isolated increases in [CO2]BL cause JHCO3 to rise whereas isolated increases in [HCO-3]BL cause JHCO3 to fall. The response to CO2 suggests the presence of a CO2 sensor near the basolateral membrane. Low-dose angiotensin II (ANG II) in either the lumen or bath accentuates the CO2 response, whereas high-dose ANG II (lumen or bath) or blockers of either tyrosine kinases or protein kinase C all markedly blunt the CO2 response. The proposed work has three aims: First, is the JHCO3 response to CO2 plastic (is it altered by chronic acid-base disturbances?), does it extend to other transporter processes, and does it extend to more tubule segments? Second, to what extent does the JHCO3 response to CO2 depend on the 'hardware' of HCO-3 reabsorption in the proximal tubule? Using specific inhibitors and knockout/transgenic mice, we will ask whether interfering with the apical Na-H exchanger (NHE3) or H+ pump or the insertion/retrieval of apical vesicles carrying these transporters reduces the response. Using mmunocytochemistry, we will ask if the CO2 response entails a change in the distribution of NHE3 or H+-pump 9roteins. Third, what signal-transduction processes are involved in the response to CO2? We will use ANG II and blockers or agonists of specific signal-transduction steps to block or mimic the effects of CO2 on JHCO3. We will examine the effects of CO2 on potential second messengers (using JHCO3 or pHi changes as indices of transport), and use 2-D gels and mass-spectroscopy to identify candidate proteins involved in signal transduction. The proposed work will provide important new insight into how the kidney responds appropriately to acid-base challenges, and will also elucidate the mechanism by which ANG II modulates PT transport.